Effect of 3 T magnetic field on RF plasma sputtering in an ITER-relevant first mirror unit

Metallic first mirrors (FMs) make up important components in many of the optical diagnostic systems in the fusion reactor ITER. They are responsible for directing the light from the fusion plasma towards the diagnostic sensors through an optical labyrinth to prevent neutron leakage. However, being the initial elements in the optical path, the FMs are subject to constant erosion from charge exchange neutrals as well as deposition of the first wall materials: beryllium (Be), tungsten (W) and their oxides, which would significantly degrade their optical properties. The FMs would hence require regular cleaning to restore their optical properties which, among other techniques, is foreseen to be achieved by in situ capacitively coupled radio-frequency (CCRF) plasma cleaning technique. In ITER, most of the mirror cleaning operations are expected to be executed in the presence of the toroidal magnetic fields in the range of 3 to 3.8 T at the locations of the FMs. The presence of fields of this magnitude would significantly influence the cold plasma properties, particularly the bias voltage on the mirror, ion directionality and flux spatial distribution, all of which can significantly affect the mirror cleaning rate and uniformity. Hence, it is crucial to investigate plasma cleaning of FMs in presence of strong magnetic fields. In this study, we experimentally investigated the influence of a 3T magnetic field on the RF discharges, wall sputtering and deposition on FMs in an ITER sized mock-up of a first mirror unit (FMU) with Cu walls. The FMU consisted of two electrodes M1 and M2 serving as the first and second mirrors, wherein M1 was powered and M2 grounded.